Adds a new field to a table.
$table: Name of the table to be altered.
$field: Name of the field to be added.
$spec: The field specification array, as taken from a schema definition. The specification may also contain the key 'initial'; the newly-created field will be set to the value of the key in all rows. This is most useful for creating NOT NULL columns with no default value in existing tables.
$keys_new: (optional) Keys and indexes specification to be created on the table along with adding the field. The format is the same as a table specification, but without the 'fields' element. If you are adding a type 'serial' field, you MUST specify at least one key or index including it in this array. See db_change_field() for more explanation why.
function db_add_field($table, $field, $spec, $keys_new = array()) {
return Database::getConnection()
->schema()
->addField($table, $field, $spec, $keys_new);
}